Numerous insect species are serious pests to common agricultural crops such as corn, soybeans, peas, cotton, and similar food and fiber crops. The primary method of controlling such pests has been through the application of synthetic chemical compounds. However, the widespread use of chemical compounds poses many problems with regard to the environment because of the non-selectivity of the compounds and the development of insect resistance to the chemicals.
Other approaches to pest control have been tried including the use of biological organisms which are typically "natural predators" of the species sought to be controlled. Such predators may include other insects, fungi, and bacteria such as Bacillus thuringiensis. Alternatively, large colonies of insect pests have been raised in captivity, sterilized and released into the environment in the hope that mating between the sterilized insects and fecund wild insects will decrease the insect population. While these approaches have had some success, they entail considerable expense and present several major difficulties. For example, it is difficult both to apply biological organisms to large areas and to cause such living organisms to remain in the treated area or on the treated plant species for an extended time. Predator insects can migrate and fungi or bacteria can be washed off of a plant or removed from a treated area by rain. Consequently, while the use of such biological controls has desirable characteristics and has met with some success, in practice these methods seem severely limited.
Advances in biotechnology in the last two decades have presented new opportunities for pest control through genetic engineering. In particular, advances in plant genetics coupled with the identification of insect growth factors and naturally-occurring plant defensive compounds or agents offer the opportunity to create transgenic crop plants capable of producing such defensive agents and thereby protect the plants against insect attack.
Transgenic plants that are resistant to specific insect pests have been produced using genes encoding Bacillus thuringiensis (Bt) endotoxins or plant protease inhibitors (PIs). Transgenic plants containing Bt endotoxin genes have been shown to be effective for control of some insects. Effective plant protection using transgenically inserted PI genetic material has not yet been demonstrated in the field. While cultivars expressing Bt genes may presently exhibit resistance to some insect pests, resistance based on the expression of a single gene might eventually be lost due to the evolution of Bt resistance in the insects. Thus, the search for additional genes which can be inserted into plants to provide protection from insect pests is needed.
Scientists have identified some specific plant components or compounds which act as defensive agents to protect a plant from attack by insect pests and pathogens. While such components are usually present at only low levels in various plant tissues, some of them are also capable of being induced to higher levels upon attack by an insect pest or a pathogen. Examples of such defensive compounds include alkaloids, terpenes, and various proteins such as enzymes, enzyme inhibitors, and lectins. Of particular interest are plant-derived compounds which can block or alter normal biomolecular activity and thus inhibit insect growth or kill the insect.
The corn rootworm (CRW) complex in the United States consists of three species, Diabrotica barberi Smith and Lawrence (Northern), D. undecimpunctata howardi Barber (Southern) and D. virgifera virgifera LeConte (Western). The western and northern species contribute the most to the economic damage to maize. The economic damage and control costs are estimated to exceed one billion dollars a year. As noted above, the major concerns of pesticide use in controlling CRW damage are its negative effect on the environment and the development of resistance by the insect. Crop rotation is becoming less effective as a CRW control method due to extended diapause in the northern CRW and the development of modified egg laying behavior in western CRW. The generation of transgenic plants with resistance to CRW could have a major economic impact. Unfortunately there are relatively few, if any, genes available that can control CRW in transgenic plants. Thus, there is a need for additional insecticidal principles, particularly those active against CRW.